1. Field of the Invention
The present invention relates to a method and apparatus for recovering oil from an oil bearing formation through injection of steam generated in situ.
2. Description of the Prior Art
Initial production from an oil well utilizes the pressure of gases in the oil formation. This is followed by pumping when the gas pressure diminishes. Eventually, even pumping is inadequate to produce acceptable quantities of oil and resort must be had to secondary recovery methods. These include thermal stimulation of the well by raising the temperature of the oil formation to lower the oil viscosity and enhance its flow.
Various types of thermal stimulation have been utilized, including electric or hot water heaters, gas burners, in-situ combustion, hot water or steam injection, and miscible phase displacement fluid such as carbon dioxide. Of these, steam injection has many advantages.
Present systems for injecting steam in deep wells have certain disadvantages. In most such systems the steam is generated at the surface and piped down through the well casing to the base of the borehole. In a deep well a considerable amount of heat is lost through the casing, and the temperature and quality of the steam is generally inadequate to effectively thermally stimulate formations at the base of the borehole.
Prior art attempts to generate steam in-situ or downhole have been ineffective since combustion requires that the fuel and air be provided at the pressure of the steam discharged from the combustor. The size and complexity of air compressors required to provide such high pressure become economically prohibitive.
An effective system of generating steam of high quality and temperature in situ is disclosed in our U.S. Pat. No. 4,243,098, issued Jan. 6, 1981, and entitled "DOWNHOLE STEAM APPARATUS". That apparatus employs a housing for location in the borehole and including a combustor for mixing and buring fuel and air, and a heat exchanger for converting circulated water into steam, the steam being injected into the borehole and the spent gases passing upwardly through the borehole to the surface. The system avoids use of high pressure air compressors, but the venting of the spent gases to the atmosphere is a source of environmental pollution which is unacceptable in certain geographical areas.
Since flooding of the oil formation with steam has been found to significantly lower the flow resistance of the oil in the vicinity of the borehole, thereby enabling extraction of the displaced oil from adjacent recovery wells, it is desirable to provide a means for in situ steam generation without attendant environmental pollution and without significant loss of heat other than in the production of steam.